What would be the way to prove that our job board is compliant with GDPR?

Often the candidate’s account can be live until they choose to close it. Under GDPR will the job board have to specify for how long a candidate’s details will be kept? How should historical records be treated?

Do we need a data protection officer? We are a job board operating outside EU but catering to needs of EU nationals as well.

When we advertise the job for the clients, do we always have to display client name on our advertising? When the candidates submit their CVs, will they have to know where their CVs is going?

Is it OK to make consent to GDPR regulations a precondition for using our job-board service?

If a candidate goes to a job board and requests to be forgotten, will the job board need to contact all of the companies that have downloaded the candidates info and make them aware?

If candidate requests to delete their data, how can we maintain a record after the deletion? Because we need at least their email id and deleted time in our records for legal purpose. Is not that a paradox?

Regarding right to delete, Is it necessary to have the option in the website itself, or we can grant those request only if candidates email us?

We own a number of job boards, each job board job alerting the the candidates opt-in to the job-alerting services, do we need to make candidates aware of each job board names?